The present invention relates to a voltage control circuit having the function of detecting which one of a reference voltage and an external supply voltage, such as a power source voltage supplied via a cable by the function of cable power status (CPS) conforming to, e.g., IEEE 1394 standard, is higher.
With the wide proliferation of multimedia centering around personal computers, the function of processing enormous amounts of data such as image data at a high speed has been required of each information device. Concurrently, technology for increasing a rate at which data is transferred between devices has been becoming important. Under such circumstances, the IEEE 1394 standard has been proposed as technology which enables high-speed data transfer, based on which development has been pursued. The IEEE 1394 standard suggests a serial transfer method whereby a transfer rate is increased by unprecedentedly reducing the number of buses, while increasing an operational frequency for transfer.
FIG. 8 shows a part of a configuration of an IEEE 1394 network system. In FIG. 8, individual network devices are connected to each other via cables. Each of the cables is composed of four data lines (a pair of data lines and a pair of strobe lines), a power source line, and a ground line. Depending on the type of a cable, only four lines compose the cable without using the power source line and the ground line.
An LSI compliant with the IEEE 1394 standard can be divided into a PHY section for primarily controlling data input and data output and a LINK section for controlling a transfer protocol. During data reception, a signal is transferred from a transfer cable to the PHY section and then from the PHY section to the LINK section. During data transmission, the signal is transferred in the opposite direction.
Besides high-speed data transfer, the IEEE 1394 standard also supports the CPS function for supplying power from a power source via the data transfer cable. The IEEE 1394 standard includes a plurality of standards such as IEEE 1394_1995, IEEE 1394. A, and IEEE 1394. B and the CPS function is defined in each of the standards.
That is, in an LSI compliant with the IEEE 1394 standard, the CPS function should be operated when the power-source voltage supplied via the cable is within a specified range. Since the CPS function is the standard for the PHY section for controlling data input and data output, it is necessary for the PHY section to have the function of detecting whether or not the power source voltage supplied via the cable is within the specified range.
In the IEEE 1394_1995, e.g., the voltage supplied from the outside by the CPS function is defined to be 8 to 40 V. This means that the CPS function should be operated when the voltage is at least in the range of 8 to 40 V. To develop an LSI compliant with the standard, therefore, it is necessary to provide the function of detecting whether or not the voltage supplied from the cable is within the range of 8 to 40 V.
However, the application of a high voltage of 8 to 40 V directly to an LSI leads to the breakdown of the LSI and is not preferred. For this reason, the standard proposes reducing a supply voltage via a 400-k xcexa9 voltage dropping resistor and applying a reduced voltage to the PHY section. Consequently, the PHY section is so configured as to estimate the supply voltage by detecting the reduced voltage so that the accuracy of voltage drop as well as the accuracy of voltage detection are important factors in observing the standard.
As far as the present inventors have investigated, however, there has been proposed no circuit configuration which conforms to the standard.
FIG. 10 shows a circuit configuration of a voltage determining unit 15 conforming to the standard, which has been devised by the present inventors. In FIG. 10, a cable 12 is connected to an input terminal 51 of a voltage control circuit 50 via a voltage dropping resistor 11 (having a resistance value R0). In the voltage control circuit 50, a resistor element 52 (having a resistance value R1) is connected to the input terminal 51 to be disposed in series to the resistor 11. A current Icps flows from the cable 12 to the ground Vss and an external voltage VDD_ext is converted to a voltage Va in the voltage control circuit 50. A comparator 53 compares the converted voltage Va with a comparison voltage Vb and outputs a signal Vcps indicative of the result of comparison. The signal Vcps becomes HIGH when the converted voltage Va is higher than the comparison voltage Vb, which enables the detection of the supply of a voltage that can be supplied via the cable as the external voltage VDD_ext.
FIG. 11 is a graph showing the relationship between the external voltage VDD_ext and the converted voltage Va in the circuit configuration of FIG. 10. In FIG. 11, a power-source voltage VDD_int to the voltage control circuit 50 is assumed to be 3.3 V and a maximum withstand voltage Vmax is assumed to be 3.6 V. As shown in FIG. 11, the external voltage VDD_ext and the converted voltage Va are constantly in a proportional relationship in the circuit configuration of FIG. 10. The resistance value R1 is adjusted to such a value that the converted voltage Va does not exceed the maximum withstand voltage Vmax of 3.6 V even when 40 V is given as the external voltage VDD_ext. Hence,
R1=3.6xc2x7R0/(40xe2x88x923.6)=0.0989xc2x7400 k=39.56 kxcexa9
is derived from
xe2x80x833.6/R1=40/(R0+R1).
If the maximum value of the converted voltage Va is set to 3.3 V by allowing a margin,
R1=3.3xc2x7R0/(40xe2x88x923.3)=0.0899xc2x7R0=39.95 kxcexa9
is satisfied. Therefore, the resistance value R1 of the resistor element 52 is preferably adjusted to be in the range of 35 kxcexa9 to 40 kxcexa9. It will easily be appreciated that the resistance value R1 of the resistor element 52 varies depending on the voltage supplied via the cable, the maximum withstand voltage, or the resistance value of the voltage dropping resistor.
Although the reference voltage serving as the standard for determining the external voltage VDD_ext is 8 V, the reference voltage has been set to 7 V and the comparison voltage Vb has been set to 0.63 V, since it is common to set the reference voltage to the order of 5 V to 7.5 V by allowing a margin. The comparison voltage Vb is set appropriately based on the reference voltage and on the ratio between the respective resistance values of the voltage dropping resistor 11 and the resistor element 52.
In a circuit configuration as shown in FIG. 10, however, the external voltage VDD_ext and the converted voltage Va are in the proportional relationship and the converted voltage Va is about {fraction (1/10)} of the external voltage VDD_ext. If the external voltage VDD_ext varies greatly, therefore, the converted voltage Va changes only by about {fraction (1/10)} of the variation of the external voltage VDD_ext. Conversely, variations in comparison voltage Vb and variations in the detection accuracy of the comparator 53 are magnified ten times to affect the detection of the external voltage VDD_ext. Variations in the resistance of the resistor 52 also greatly affect the accuracy with which the external voltage VDD_ext is detected.
On the other hand, it is considered that an input to the comparator is preferably set to about xc2xd of the power-source voltage in terms of sensitivity. In the circuit configuration of FIG. 10, the level of the input to the comparator 53 is close to the ground potential Vss, which is not preferred in terms of detection accuracy.
It is therefore an object of the present invention to perform a high-accuracy comparison between an external voltage and a reference voltage in a voltage control circuit for implementing, e.g., the CPS function. To attain the object, the present invention has widened the range of a converted voltage in a region in which the external voltage is close to the reference voltage and shifted the range to higher voltages, thereby increasing the accuracy of comparison between the external voltage and the reference voltage.
Specifically, the present invention in a first aspect is a voltage control circuit for detecting which one of an external voltage supplied from the outside and a reference voltage serving as a standard for comparison is higher, the circuit comprising: an input terminal to which the external voltage is applicable via a voltage dropping resistor; voltage converting means for converting, in conjunction with the voltage dropping resistor, the external voltage applied to the input terminal to a voltage lower than the external voltage; and comparing means for comparing a converted voltage outputted from the voltage converting means with a specified comparison voltage corresponding to the reference voltage and outputting a signal indicative of the result of comparison as a signal indicative of which one of the external voltage and the reference voltage is higher, the voltage converting means performing a voltage conversion such that a ratio of an increment of the converted voltage to an increment of the external voltage is relatively high in a comparison region in which the external voltage is close to the reference voltage and relatively low in a region in which the external voltage is higher than in the comparison region.
In accordance with the present invention in the first aspect, the voltage converting means performs a conversion such that the ratio of the increment of the converted voltage to the increment of the external voltage is relatively high in the comparison region and relatively low in a region in which the external voltage is higher than in the comparison region. This provides a wide range of converted voltages in the comparison region, while controlling the converted voltage in a region in which the external voltage has a value higher than in the comparison region such that the converted voltage does not exceed the withstand voltage. This remarkably suppresses the influence given by the varying comparison voltage and the varying detection accuracy of the comparing means on the external voltage, so that the accuracy of a comparison made between the external voltage and the reference voltage is increased.
Preferably, the voltage converting means in the voltage control circuit comprises a diode or diode-connected transistor connected to the input terminal to be disposed in series to the voltage dropping resistor. Preferably, the comparing means in the voltage control circuit comprises comparison voltage generating means for generating the comparison voltage, the comparison voltage generating means comprising: a constant current source; and a diode or diode-connected transistor disposed in series to the constant current source.
Alternatively, the voltage converting means in the voltage control circuit comprises: a voltage dropping element connected to the input terminal to be disposed in series to the voltage dropping resistor; and a clamping circuit for limiting a voltage on a side of the voltage dropping element closer to the input terminal such that the voltage does not exceed a specified limit voltage.
Preferably, the clamping circuit comprises: a transistor disposed in parallel with the voltage dropping element; and a comparator for receiving a drain voltage of the transistor as one input and the limit voltage as the other input, an output of the comparator serving as an input to a gate of the transistor. Alternatively, the clamping circuit comprises a single diode or diode-connected transistor disposed in parallel with the voltage dropping element or a plurality of diodes or diode-connected transistors disposed in series to each other and in parallel with the voltage dropping element. Preferably, the voltage dropping element is a resistor, a diode, or a diode-connected transistor.
In a second aspect, the present invention is a network device connectable to a cable, the device comprising a voltage determining unit for determining whether or not an external supply voltage supplied via the cable is within a specified range, the voltage determining unit comprising: a voltage control circuit as recited in claim 1; and a voltage dropping resistor having one end connected to the input terminal of the voltage control circuit and the other end to which the external supply voltage is given.
In a third aspect, the present invention is a voltage control circuit for detecting which one of an external voltage supplied from the outside and a reference voltage serving as a standard for comparison is higher, the circuit comprising: an input terminal to which an external voltage is applicable via a voltage dropping resistor; a resistor element having one end connected to the input terminal and the other end grounded; and a comparator for comparing a voltage at the one end of the resistor element with a specified comparison voltage corresponding to the reference voltage and outputting a signal indicative of the result of comparison as a signal indicative of which one of the external voltage and the reference voltage is. higher.
Preferably, the voltage dropping resistor in the voltage control circuit has a resistance value of 400 kxcexa9 and the resistor element has a resistance value of 35 kxcexa9 to 40 kxcexa9.
Preferably, the specified comparison voltage is determined based on the reference voltage and on a ratio between respective resistance values of the voltage dropping resistor and the resistor element.
In a fourth aspect, the present invention is a method of detecting, in a network device, which one of an external supply voltage supplied via a cable and a reference voltage serving as a standard for comparison is higher, the method comprising the steps of: converting the external supply voltage to a voltage lower than the external supply voltage; and comparing a converted voltage obtained in the voltage converting step with a specified comparison voltage corresponding to the reference voltage, the voltage converting step performing a voltage conversion such that a ratio of an increment of the converted voltage to an increment of the external supply voltage is relatively high in a comparison region in which the external supply voltage is close to the reference voltage and relatively low in a region in which the external supply voltage is higher than in the comparison region.